The biogenic production of volatile compounds which cause objectionable odors is one of the problems associated with the collection and treatment of various waste materials. Domestic sewage is the largest source of such odorous compounds. Various reduced sulfur-containing compounds are common, with hydrogen sulfide being the most objectionable odor-causing compound in such wastes. Because of the magnitude of domestic sewage that is collected and treated and the prominence of the associated odorous sulfidic compounds, the present invention is particularly directed, but not limited to the control of hydrogen sulfide and other sulfide odors in municipal or industrial waste. As used herein, the term “sulfidic compounds” also includes hydrogen sulfide (H2S), mercaptans (RSH), and other related odoriferous sulfidic compounds.
The mixed biological population common to municipal or industrial waste utilizes the compounds found in the waste as a source of nutrient. In this process, oxygen is the preferred terminal electron acceptor, and the nutrient, commonly an organic compound, is oxidized. In highly nutrient loaded systems such as municipal sewage, bacterial action can result in a rapid consumption of oxygen in the water. In the absence of oxygen, bacteria require an alternate terminal electron acceptor.
In general, bacteria will utilize the terminal electron acceptor that provides them with the greatest amount of energy. Thus, there is a preferred selection order of a terminal electron acceptor by bacteria. This order is shown below.O2>NO3−>Fe>SO4−2>CO3−2
As nitrate is not typically found in natural waters, the sulfate ion (SO4−2) is generally the preferred alternate. In the absence of oxygen, unless nitrate is added supplementally, those bacteria which can utilize sulfate as a terminal electron acceptor in their respiration process will predominate. The most well-characterized bacteria of this type is Desulfovibrio desulfuricans, and is most commonly referred to as sulfate-reducing bacteria, SRB. SRB are known to metabolize sulfate ion with organic matter to form H2S as shown in the following equation.SO4−2+ organic matter+SRB→H2S+CO2+H2OH2S, responsible for the characteristic odor from rotten eggs, is toxic in low concentrations. Citizen complaints are often the driving force behind efforts to control odor. Such odors are generally regarded as a public nuisance and a health hazard.
Although H2S is a gas, H2S in water can dissociate with increasing pH as shown in the following equations. Thus at a given pH, the relative amount of dissolved H2S species can be predicted. The sulfide ion, S−2, and bisulfide ion, HS−, being ionic, are constrained to remain in the aqueous phase.H2S+OH−→HS−+H2OHS—+OH−→S−2+H2O(H2S—gas phase & aqueous phase, HS− & S−2 aqueous phase)
Description of Figure
The figure herein shows the relationship between these species, the evolution of the gas from aqueous solution being a function of pH. At the pH typically found in sewer systems, a significant percent of the H2S formed evolves from solution. The gas can redissolve on the crown of the sewer line, and the presence of Thiobacillus bacteria and others, metabolize the H2S, producing sulfuric acid, H2SO4. This can and has resulted in sewer line collapse and results in a significant cost in terms of their repair and replacement.
H2S is also corrosive towards steel and concrete. H2S is a gas, with the evolution of the gas from aqueous solution being a function of pH. At pHs often found in sewer systems, a significant percent of the H2S formed evolves from solution. The gas can redissolve on the crown of the sewer line, and the presence of Thiobacillus bacteria and others, metabolize the H2S, producing sulfuric acid, H2SO4. This can and has resulted in sewer line collapse and results in a significant cost in terms of their repair and replacement.
Various compounds, including hypochlorite (sodium or calcium), potassium permanganate, sodium nitrate, ferrous and ferric salts, hydrogen peroxide, chlorine, chlorine dioxide, and sodium chlorite have been widely used for the control of odor in wastes, and sewage waste in particular.